How to stop closing a page with unsaved changes.

function beforeUnloadListener(event) { event.preventDefault(); return event.returnValue = 'Are you sure you want to exit?'; };

// A function that invokes a callback when the page has unsaved changes. onPageHasUnsavedChanges(() => { window.addEventListener('beforeunload', beforeUnloadListener); });

// A function that invokes a callback when the page's unsaved changes are resolved. onAllChangesSaved(() => { window.removeEventListener('beforeunload', beforeUnloadListener); });